A nuclear power cycle may be viewed as originating with a nuclear reaction which occurs within a component commonly called a "nuclear reactor." A nuclear reactor typically comprises a vessel which defines a chamber and a fuel core which is situated within this chamber. In commercial reactors for utility applications, the fuel core consists of a number of fuel rods which are arranged so that a self-sustaining nuclear reaction will take place. This nuclear reaction releases an enormous amount of energy which is used to transfer heat to a circulating fluid.
In one type of nuclear reactor, a "boiling water reactor" (BWR), the circulating fluid is water which is converted to steam within the reactor vessel and then supplied to the turbine of the power cycle. In another type of nuclear reactor, a "pressurized water reactor" (PWR), the circulating fluid is heated within the reactor vessel and then supplied to a closed heat exchanger, or steam generator, wherein the circulating fluid converts water to steam which is then supplied to the turbine of the power system. In either case, the reactor vessel will include input/output nozzles for the circulating fluid which are connected to the appropriate plant lines. Additionally, both BWR units and PWR units will include reactor internals which interact with the circulating fluid and/or which are used to control the fuel core.
As was indicated above, an enormous amount of energy is released during a nuclear reaction. In fact the amount of energy released per atom exceeds by a factor of several million the amount of energy obtainable per atom in a chemical reaction, such as the burning of fossil fuels. Consequently, nuclear power has become a very attractive alternative for the utility industry and more than 500 nuclear reactors are now either on-line or under construction world-wide. However, the nuclear reaction also emits potentially harmful radiation thereby creating sometimes complex construction and operating challenges. Additionally, and of particular interest in the present application, this radiation produces certain obstacles when dismantling or decommissioning a nuclear reactor at the end of its operating life. More specifically, the inside of the vessel, along with the reactor internals, will be considered radioactively contaminated thereby placing certain restrictions on the decommissioning of the reactor.
It should be noted at this point that removal and disposal of a nuclear reactor's fuel core is not normally considered part of the decommissioning process. This is due to the fact that it is an accepted industry practice to replace one-third of the fuel rods of a nuclear reactor each operating year. Consequently, the removal/disposal of the fuel core during a decommissioning process will usually not present any problems above and beyond those encountered during annual replacements. However, the decommissioning of the remaining portions the nuclear reactor, i.e., the vessel and the reactor internals, presents challenges not normally confronted during the maintenance of an on-line nuclear reactor.
In the past, the decommissioning of nuclear reactors has been accomplished almost exclusively by a "water platform" method. In such a method, the top of the reactor vessel is removed and the vessel is filled with water which thereby functions as a radioactive shield relative to the reactor internals. A platform is placed on top of the water and underwater cutting is performed on the pieces of the reactor internals located immediately beneath the platform. These pieces are then loaded onto the platform and transferred to either a "wet cutting station" in which further underwater cutting is performed or a "dry cutting station" in which further cutting is performed in an air-controlled environment. The resulting pieces of the reactor internals are then placed in "casks" which comprise a casing made of a radioactively shielding material. The water level is then decreased, the platform lowered, and the cutting process is again initiated. This sequence of events is repeated until all of the reactor internals have been removed. Thereafter the cutting of the reactor vessel itself is initiated.
This "water platform" method of decommissioning a nuclear reactor, while acceptably effective, places many time, cost and safety constraints on a decommissioning project. For example, although water functions as a radioactive shield, some worker interaction with the sectioned reactor internals will usually be experienced in the transfer between the water platform and the wet/dry cutting stations. Additionally, the cutting process usually produces a significant amount of particles whereby respirators and HEPA ventilation are sometimes necessary to combat the effects of airborne contamination. Furthermore, the "shielding" water in the vessel will absorb particles produced during the cutting process whereby constant circulation and filtration of this fluid is necessary to remove liquid radioactive waste.
An alternate method of decommissioning a nuclear reactor was recently used on a retired nuclear reactor at the Shippingport Power Plant. The Shippingport nuclear reactor was an offspring of the Eisenhower presidency and thus its decommissioning was orchestrated by the United States Department of Energy. In decommissioning this unit, the reactor vessel was filled with concrete and then moved in one piece to a disposal site. The Shippingport reactor, which was rated at 72 megawatts, was significantly smaller than most of the commercial nuclear reactors in use or construction today. Nonetheless, the weight of the reactor when filled with concrete required the fabrication of special lifting equipment to lift the reactor from its underground housing. More particularly, the project required the erection of a gigantic frame, the construction of four huge hydraulic jacks, each having an approximately 6,000 ton lifting capacity, and the mounting of these jacks on the frame. In the transfer of the Shippingport reactor to the transport vehicle (a barge in this case), the jacks hoisted the reactor seventy-seven feet into the air, moved it approximately thirty-eight feet horizontally along a track and then lowered it onto a trailer.
The Energy Department's decision to decommission the Shippingport reactor in this manner, which avoided cutting apart the radioactive structure, saved an estimated seven million dollars and, perhaps more importantly, dramatically reduced worker exposure to radiation. However, such a procedure is probably not possible for most commercial reactors which possess an average rating of approximately 1000 megawatts, and would weigh over 2500 tons if filled with concrete. Moreover, even if larger reactors could be moved in one piece, the capital cost of fabricating the necessary lifting equipment would probably make such an approach economically unfeasible.
Applicant therefore believes that a need remains for a cost effective method of decommissioning a nuclear reactor in which radiation exposure is minimized.